1. Field of the Invention
The present invention relates to digital data storage, and more particularly, to techniques for recovering from shock events occurring to a disk drive during data write operations to improve data reliability.
2. Description of the Prior Art
Market demand for increased storage capacity has pushed disk drive designers to increase the drive""s track density, often expressed as tracks per inch or TPI. Due to the increased track density, an external shock event may undesirably force the disk drive""s read/write head from the desired track during track following and cause the head to vibrate about the track""s centerline. The head vibrations may be sufficient large to impact the reliability of stored data.
Accordingly, there exists a need for a technique that enables a disk drive to recover from a shock event in a manner that improves the reliability of the stored data.
The present invention may be embodied in a method performed by a disk drive for recovering from shock events to improve data reliability. The disk drive includes a read/write head and a rotating disk having a data storage surface. The storage surface has a plurality of embedded servo wedges and a plurality of concentric data tracks with data sectors. The disk drive further includes an on-track state wherein writing of data is enabled, a write unsafe state wherein writing of data is disabled, and a shock recovery state wherein writing of data is disabled. In the method, an arrival threshold, a write unsafe threshold and a shock detection threshold are provided. The write unsafe threshold is greater than the arrival threshold, and the shock detection threshold is greater than the write unsafe threshold. The embedded servo wedges on the rotating disk are read to generate position error signal values with respect to a selected data track. The disk drive is transitioned from the on-track state to the write unsafe state if a position error signal value is greater than the write unsafe threshold and less than the shock detection threshold. The disk drive is transitioned from the on-track state or from the write unsafe state to the shock recovery state if a position error signal value is greater than the shock detection threshold. The disk drive is transitioned from the shock recovery state to the write unsafe state if a first number of consecutively generated position error signal values remain less than the write unsafe threshold. Also, the disk drive is transitioned from the write unsafe state to the on-track state if a second number of consecutively generated position error signal values remain less than the arrival threshold.
In more detailed features of the invention, the first number may be based on the magnitude of a position error signal value that exceeds the shock detection threshold. The first number may be greater than about one-half of the total number of servo wedges per track and less than about the total number of servo wedges per track. For example, the first number may be equal to or greater than about ninety, and the second number may be a predetermined number that is equal to about seven. The centers of the data tracks are separated by a track-to-track spacing, and the shock detection threshold may be about 30% to 40% of a value associated with the track-to-track spacing, the write unsafe threshold may be about 16% to 18% of a value associated with the track-to-track spacing, and the arrival threshold may be about 11% to 14% of a value associated with the track-to-track spacing. Thus, the shock detection threshold may be about twice the write unsafe threshold.
Another embodiment of the invention may reside in a method that likewise improves data reliability. In the method, each position error signal value is monitored during the data write operation. If a monitored position error signal value exceeds a write unsafe threshold, then the following steps are performed. Data writing is disabled. If the monitored position error signal value further exceeds a shock event threshold, which threshold is greater than the write unsafe threshold, then the disk drive waits for a first number of consecutively generated position error signal values that are less than the write unsafe threshold. The disk drive waits for a second number of consecutively generated position error signal values that are less than an arrival threshold, which threshold is less than the write unsafe threshold. The disk drive then enables writing of data.